A major limitation of current techniques of culturing epithelial cells in vitro is that the cells rest on plastic rather than basement membrane or within a tissue matrix. This research focuses on routine use of the extracellular matrix (ECM) produced by cultured bovine endothelial cells as a naturally produced substrate most adequate for sustaining adhesion, growth and differentiation of epithelial cells. Using this substrate, a routine procedure has been developed for the establishment in culture of actively dividing normal primary and secondary human mammary epithelial cells free of stromal fibroblasts. The high (80-100%) rate of success resulted from the combined use of a serum-free medium supplemented with high-density lipoprotein and of cell plating on ECM. In preliminary studies with carcinomatous material derived from the same mastectomized breast, plating on ECM and the addition of epidermal growth factor were found essential for the initiation of a monolayer culture. Experiments are under way to adapt the above culture conditions to grow human bladder and ovary carcinoma and to improve the culture conditions so as to allow cell proliferation at low and clonal seeding levels. Expression of differentiation functions by cell plating on ECM was demonstrated by the induction of neuronal differentiation in pheochromocytoma, oligodendrocytes and dorsal root ganglia cells as well as by the long-term maintenance of hormone-secreting-and-responding rat pituitary cells in primary cultures. Subjecting the ECM to various enzymatic and chemical modifications yielded differential effects on its ability to induce cell attachment, proliferation and differentiation, thereby indicating that different components of the ECM are responsible for its induction of specific biological activities. Studies on the interaction of melanoma and lymphoma cells with the vascular endothelium and its underlying basal lamina have demonstrated a correlation between their metastatic potential in vivo and ability to degrade heparan sulfate in the subendothelial ECM by means of a specific heparitinase activity. This activity also has been demonstrated in activated normal macrophages and T\lymphocytes and in platelets. The involvement of platelets in tumor cell metastasis and the protective role of heparin are currently being investigated.